Hydraulic power unit

ABSTRACT

A hydraulic power unit from which a hydraulic fluid may be discharged under pressure to the wing control actuators of a guided missile, the actual pressure at which such fluid is discharged during operation being changed in accordance with the requirements of the wing control actuators. In one of two embodiments disclosed the power unit includes two plenum chambers for the hydraulic fluid, means for independently varying the pressure on the hydraulic fluid in each one of such chambers using a source of substantially constant pressure for such purpose, and means for connecting the wing control actuators to either one of the plenum chambers as required during the flight of the guided missile.

United States Patent Estlick [52] U.S. Cl ..60/54.6 A, 60/54.6 P, 92/6R, 92/62 [51] Int. Cl ..Fl5b 7/00,F01b31/00, FOlb 7/00 Field of Search..60/54.6 P, 54.5 R, 10.5, 54.6 S, 60/545 A, 54.6 A, 54.5 HA, 54.6 HA;92/6, 52,62,

[56] References Cited UNITED STATES PATENTS Alping 2.92/52 Brent et a1...60/54.6 A

CONTROL ACTUATOR GUIDANCE 51 July 18,1972

Oswalt ..60/54.6 A Lagerquist ..60/54.6 A

[57] ABSTRACT A hydraulic power unit from which a hydraulic fluid may bedischarged under pressure to the wing control actuators of a guidedmissile, the actual pressure at which such fluid is discharged duringoperation being changed in accordance with the requirements of the wingcontrol actuators. In one of two embodiments disclosed the power unitincludes two plenum chambers for the hydraulic fluid, means forindependently varying the pressure on the hydraulic fluid in each one ofsuch chambers using a source of substantially constant pressure for suchpurpose, and means for connecting the wing control actuators to eitherone of the plenum chambers as required during the flight of the guidedmissile.

12 Claims, 2 Drawing Figures GATE VALVE VALVE SIG CONTROLLPATiNn-inauuemz 77,0 5

SHEET 1 [IF 2 WING CONTROL ACTUATOR GUIDANCE SIGNALS VALV E CONTROLINVENTO/P RAYMOND J- ESTL/CK PATENTED JUL18|972 3.671005 SHEET 2 OF 2FIG. 2

lNVE/VTOR RAYMOND J. ESTL/CK HYDRAULIC POWER UNIT BACKGROUND OF THEINVENTION This invention relates generally to hydraulic power units andparticularly to power units wherein high pressure storage gas is used todischarge a hydraulic fluid stored in a plenum chamber within such unit,such fluid being used to provide hydraulic power to a load such as thewing control actuator for a guided missile. In such an application, themissile typically is maneuvered in accordance with the aerodynamicresponse of the missile to the angular deviation of wings mounted on themissile, the desired angular deviation being controlled by a hydraulicwing actuator servo responsive to the guidance command signals. In theusual case the guidance commands require that the missile makerelatively violent maneuvers at the launch and terminal phases of itsflight and relatively moderate maneuvers during the midcourse phase ofits flight. The large maneuvers during the launch phase are required tocorrect for initial launching angle errors while the large maneuversduring the terminal phase are required to enable the missile tooutmaneuver the target just prior to intercept. Known hydraulic wingactuator servos employ a high pressure stored gas operating on a movablewall of a plenum chamber to discharge a hydraulic fluid to operate suchactuators. As the stored fluid is ejected from the plenum chamber, thestored gas expands to maintain pressurization of the fluid. Inmaintaining such high level pressurization throughout the missilesflight it has been found that an excessive amount of the potentialenergy of the stored gas is expended unnecessarily by response of thewing actuator servo to various noise disturbances superimposed on theguidance command signals during the midcourse phase of flight.Therefore, adequate gas pressure may not be available for the largemaneuvers generally required of the missile during its terminal phase offlight.

SUMMARY OF THE INVENTION It is therefore an object of the invention toprovide a hydraulic power unit wherein potential energy of the gasstored within such unit is utilized in a most efficient manner.

It is an object of the invention to provide a hydraulic power unitwherein the duty cycle of such unit is matched to the duty cycle of ahydraulic device coupled to such unit.

It is an object of the invention to provide a hydraulic power unitwherein the fluid contained within such unit is supplied to a hydraulicdevice, such fluid being supplied at a pressure level commensurate withthe pressure level requirement of such device, thereby to utilize thepotential energy of the stored gas in the most efficient manner.

These and other objects of the invention are accomplished by providing ahydraulic power unit for supplying a hydraulic fluid stored in a plenumchamber to a hydraulic device coupled thereto, the pressure on thehydraulic fluid being matched to the requirements of the hydraulicdevice, thereby conserving the potential energy of the stored gas. Thedesired matching is accomplished by sequentially varying the effectivearea of a piston operating on the hydraulic fluid in the plenum chamberwithout changing the area on which the stored gas operates.

BRIEF DESCRIPTION OF THE DRAWINGS The aforementioned objects and otherfeatures of the invention are explained in the following descriptiontaken in connection with the accompanying drawings wherein:

FIG. 1 is an isometric view of a preferred embodiment of the invention,partially cut away and somewhat distorted, to shown the features of theinvention; and

FIG. 2 is an alternative embodiment of the invention, greatlysimplified, to illustrate the principles of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 it maybe seen that a wing control actuator 10, which may, for example, be aconventional double acting hydraulic ram, receives hydraulic fluid (notnumbered) through either lines 12 or 14, depending on the condition ofservo valve 16. The latter element is responsive to control signals froma valve controller 18 to permit operation of the wing control actuator10 in either direction. The hydraulic fluid supplied to the wing controlactuator 10 is vented through line 20 and a gate valve 22 after adesired maneuver is completed. Gate valve 22 may be a conventionalsolenoid valve so that it may be opened and closed in response to asignal from valve controller 18 to control venting. The servo valve 16is connected as shown to a distribution chamber 24, such chamber beingsupplied hydraulic fluid from hydraulic power unit 26. For reasons tobecome clear, gate valves 28 and 30 provide the control means forenabling hydraulic fluid to enter distribution chamber 24 selectivelyfrom either port 32 or port 34.

The hydraulic power unit 26 includes a casing 36, such casingenclosing agas cartridge 38, slidably mounted within the casing 36, the outersurface of such cartridge slidably supporting piston 40. Pistons 40, 42are annular in shape, piston 40 extending as indicated between the outersurface of gas cartridge 38 and the inner wall (not numbered) of casing36, and piston 42 extending as indicated between the outer surface 44 ofend cap 46 and the inner wall of casing 36. Portions of pistons 40, 42,inner wall of casing 36, outer surface 44 of end cap 46 and gascartridge 38 define, as shown, the walls of a pair of plenum chambers48, 50 for hydraulic fluid.

Plenum chambers 48, 50 are filled with hydraulic fluid via fill valves52 and 54. Gas cartridge 38 is tilled with gas, preferably helium, viafill valve 56. To initiate the launch phase of operation a signal istransmitted to gate valve 28 by valve controller 18. Such transmittedsignal opens gate valve 28 whereby gas contained within gas cartridge 38flows through pressure regulator valve 57 and into annular space 58,thereby forcing pistons 40, 42 to move apart. It is noted, however, thatbecause gate valve 30 is closed during the launch and midcourse phasesof operation, piston 42 moves but slightly, if at all, due to theincompressible nature of the hydraulic fluid in plenum chamber 50. It isobvious, however, that the hydraulic fluid in plenum chamber 50 ispressurized. The piston 40 during its initial movement similarly causesthe hydraulic fluid in plenum chamber 48 to be pressurized and suchfluid is discharged via port 32 through distribution chamber 24 andthrough either line 12 or 14 (depending on the condition of servo valve16) to wing control actuator 10. It follows then that hydraulic fluidunder pressure is forced from plenum chamber 48 as piston 40 continuesto move. When piston 40 engages with gas cartridge 38 by means of rim60, both then move together, continuing to force hydraulic fluid out ofplenum chamber 48 until piston 40 and gas cartridge 38 rest on casing36. A moments thought will make it clear, however, that after piston 40engages gas cartridge 38 the pressure on the hydraulic fluid drops to alower level than it was prior to such engagement. The reason for thedrop in pressure is that: l the force (F) exerted on piston 40 by thegas from gas cartridge 38 is the same before and after engagement ofpiston 40 with gas cartridge 38; (2) prior to such engagement thepressure on the hydraulic fluid in plenum chamber 48 is P, F/A (where Ais the pressurizing area of piston 40); and (3) after such engagementthe pressure on such hydraulic fluid is P F/A (where A A plus thepressurizing area of gas cartridge 38)..

The tenninal phase of operation is initiated by the valve controllertransmitting a signal to gate Valve 30. Such signal opens gate valve 30,thereby releasing hydraulic fluid from plenum chamber 50 to port 34 vialine 62. Since the force (F) on piston 42 is nearly equal to that onpiston 40, and since the pressurizing area (A of piston 42 is smallerthan the pressurizing area A of the engaged piston 40/gas cartridge 38,the

desirable leaks of hydraulic fluid or gas.

Referring now to FIG. 2, the here contemplated hydraulic power unitincludes a gas cartridge 76, a piston 74, a casing 78 and an end cap 80.Gas cartridge 76 has a gas flow annulus 90 formed therein and alongitudinal rod 94 affixed thereto. Such rod has a notch 96 formedtherein as shown. Casing 78 has an aperture (not numbered) formedtherein for receiving longitudinal rod 94. Piston 74, mounted betweengas cartridge 76 and casing 78, is in slidable engagement therewith andhas a rearward extending sleeve 86 with a notch 88 formed therein. Gascartridge 76 and piston 74 are sealed by end cap 80, such end cap beingthreadably engaged with casing 78. A sleeve locking mechanism 100,mounted to gas cartridge 76, comprises a spring loaded piston 102, afluid receiving chamber 104 and an input port 106. A cartridge lockingmechanism 108, mounted to casing 78, comprises a piston 110, an inputport 112, a fluid receiving chamber 114, an output port 116 and anexhaust port 118. A tube 120 connects output port 116 to input port 106.A distribution chamber 122, formed within casing 78, is used to couplethe hydraulic power unit to a load (not shown), such chamber havingincluded therein a spring loaded check valve (not numbered).

Having assembled the casing 78, piston 74, gas cartridge 76 and end cap80, a stop cap 126, the diameter of such cap being larger than theaperture (not numbered) is threaded to rod 94. A plenum chamber 98 isformed within the hydraulic power unit, such chamber being defined byportions of piston 74, casing 78 and gas cartridge 76 as shown. Gas,preferably helium, is introduced into gas cartridge 76 by a gas fillvalve (not shown). Hydraulic fluid is introduced into plenum chamber 98by a conventional valve (not shown). In operation, as the stored gaswithin gas cartridge 76 is introduced into the gas flow annulus 90 by aconventional gas release and regulator valves (not shown) such gaspasses through hole 89, forcing piston 74 to slide and thereby dischargehydraulic fluid contained within plenum chamber 98 through distributionchamber 122. The pressure level of the fluid discharged from plenumchamber 98 is determined by the force (F) of the gas on piston 74 andthe pressurizing area (A,) of such piston. Piston 74 continues to slideon gas cartridge 76 until notch 88 passes over piston 102 of sleeveholding mechanism 100. The spring loaded piston 102 engages piston 74 tothereby lock the gas cartridge to the piston 74. As piston 74 and gascartridge 76 slide together in response to the force of the gas, thepressure of the hydraulic fluid passing through distribution chamber 102drops to a lower pressure level. This lower pressure level hydraulicfluid flow continues until such time as slot 96 passes under cartridgelocking mechanism 108 so that piston 110 is forced into slot 96 byhydraulic fluid entering fluid receiving chamber 114 via port 112. Thegas cartridge 76 is thereby locked to casing 78 while essentiallysimultaneously piston 102 is disengaged from notch 88 by hydraulic fluidflowing via port 116 and tube 120 into fluid receiving chamber 104.After such disengagement gas cartridge 76 continues to slide in responseto force of the gas; however, since the effective piston areapressurizing the hydraulic fluid in phenum chamber 98 is reduced, suchfluid is delivered through distribution chamber 122 at a high pressurelevel. Gas cartridge 76 is restrained from reacting to this highpressure level fluid by piston 110.

O-ring seals 128 are appropriately incorporated into the variouscomponents of the hydraulic power unit to prevent undesirable leaks offluid or gas. The exhaust port 118 of bottle lockinG mechanism 108 isprovided to exhaust hydraulic oil and thereby prevent hydraulic lock insuch mechanism.

It will be obvious to one of ordinary skill in the art that the mannerof varying the size of the walls of the plenum chamber may be by othermeans than the simple piston arrangements discussed and that the meansfor applying a force to a portion of the walls of the plenum chamber maybe by means other than gas, as for examPle a spring mechanism Therefore,the form of the invention described above should be considered asillustrated and not as limiting the scope of the following claims.

What is claimed is:

1. A hydraulic power unit for discharging hydraulic fluid at varyingpressure levels to a hydraulic device coupled to such power unit,comprising:

a. a plenum chamber for storing hydraulic fluid to be discharged, suchplenum chamber being contractile and having a discharge port, suchdischarge port providing at least a part of the coupling between theplenum chamber and the hydraulic device;

b. means for contracting the plenum chamber by applying a substantiallyconstant force to a portion of a wall partially defining such chamberwhereby hydraulic fluid is discharged at a first pressure level from theplenum chamber to the hydraulic device through the discharge port; and

c. engagement means to connect said portion with the remainder of saidwall to vary the size of said portion of the wall of the plenum chamberas such plenum chamber contracts whereby hydraulic fluid is dischargedat a varying pressure level from the plenum chamber to the hydraulicdevice through the discharge port.

2. A hydraulic power unit for discharging hydraulic fluid at varyingpressure levels to a hydraulic device coupled to such power unit,comprising:

a. a plenum chamber for storing hydraulic fluid to be discharged, suchplenum chamber being contractile, partially defined by first and secondslidable pistons and having a discharge port, such discharge portproviding at least a part of the coupling between the plenum chamber andthe hydraulic device;

b. means for sliding, with substantially constant force in a directionto contract the size of the plenum chamber, the first slidable pistonthrough a first and second distance;

c. means for connecting the first slidable piston to the second slidablepiston during the second distance.

3. A hydraulic power unit for discharging hydraulic fluid at varyingpressure levels to a hydraulic device coupled to such power unit,comprising:

a. a plenum chamber for storing hydraulic fluid to be discharged, suchplenum chamber being contractile, partially defined by a wall and havinga discharge port formed therein, such discharge port providing at leasta part of the coupling between the plenum chamber and the hydraulicdevice;

b. a plurality of pistons forming such wall of the plenum chamber;

c. meanS for contracting the plenum chamber by applying a substantiallyconstant force to one of such pistons whereby hydraulic fluid isdischarged at a first pressure level from the plenum chamber to thehydraulic device through the discharge port; and

d. means for interconnecting selected ones of such plurality of pistonsas such plenum chamber contracts whereby hydraulic fluid is dischargedat corresponding selectively varied pressure levels from the plenumchamber to the hydraulic device through the discharge port.

4. A hydraulic power unit for discharging hydraulic fluid at varyingpressure levels to a hydraulic device coupled to such power unit,comprising:

a. a plenum chamber for storing hydraulic fluid to be discharged, suchplenum chamber being contractile and having a discharge port, suchdischarge port providing at least a part of the coupling between theplenum chamber and the hydraulic device;

b. first and second slidable pistons forming a wall of the plenumchamber;

c. means for sliding the first slidable piston with substantiallyconstant force in a direction to contract the size of the plenum chambera first, second and third portion;

d, means for connecting the first slidable piston to the second slidablepiston during the second portion of the contraction of the plenumchamber; and,

. means for disconnecting the first slidable piston from the level fromthe plenum chamber to the hydraulic device through the discharge port;and

. engagement means to connect said portion with the remainder of suchwall to vary the size of said portion of the wall of the plenum chamberas such plenum chamber contracts whereby h draulic fluid is dischargedat a varymg pressure level rom the plenum chamber to the hydraulicdevice through the discharge port.

. The hydraulic power unit recited in claim 7 wherein: the wall iscomprised of a first and second piston, each such piston being slidablewithin such housing;

. the contracting means includes means for sliding the first piston,with substantially constant force in a direction to contract the size ofthe plenum chamber, through a first and second distance; and

5. A hydraulic power unit for discharging hydraulic fluid at varyingpressure levels to a hydraulic device coupled to such power unit,comprising:

a. a first plenum chamber for storing a first portion of c. theengagement means includes means for connecting the first piston to thesecond piston as such first piston slides from the first distance to thesecond distance.

hydraulic fluid to be discharged, such plenum chamber being contractileand having a first discharge port, such first discharge port providingat least a part of the coupling between the plenum chamber and thehydraulic device;

. means for contracting the first plenum chamber by applying asubstantially constant force to a portion of the wall of the plenumchamber;

. means for varying the size of said portion of the wall of the firstplenum chamber;

. a second plenum chamber for storing a second portion of means forcontracting the second plenum chamber by applying a substantiallyconstant force to a portion of the wall of the second plenum chamber;and

. means connected to the first discharge port of the first plenumchamber, the second discharge port of the second plenum chamber, and thehydraulic device for selecting which one of such is coupled to thehydraulic device.

9. The hydraulic power unit recited in claim 7 wherein:

a. the wall is comprised of a plurality of pistons;

b. the contracting means includes means for applying a substantiallyconstant force to one of such pistons whereby hydraulic fluid isdischarged at a first pressure level from the chamber to the hydraulicdevice through the discharge port; and

c. the engagement means includes means for interconnecting selected onesof such plurality of pistons as such plenum chamber contracts wherebyhydraulic fluid is discharged at corresponding varied pressure levelsfrom the plenum chamber to the hydraulic device through the dischargeport.

10. The hydraulic power unit recited in claim 8 wherein the area of thewall defined by the first piston is smaller than the area of the walldefined by the second piston.

11. A hydraulic power unit for discharging hydraulic fluid at a varyingpressure level to a hydraulic device coupled to such power unit,comprising:

a. a housing, such housing having formed therein a first and secondplenum chamber; i. such first plenum chamber having stored therein afirst portion of hydraulic fluid to be discharged, such first plenumchamber being contractile, partially defined by a first wall and havinga first discharge port, such first discharge port providing at least apart of the coupling between the first plenum chamber and the hydraulicdevice; and

6. A method for discharging hydraulic fluid at varying pressure levelsthrough a discharge port formed within a contractile fluid storingplenum chamber, the steps comprising:

a. contracting the plenum chamber by applying a substantially constantforce to a portion of a wall partially defining the plenum chamberwhereby hydraulic fluid is discharged at a first pressure level from theplenum chamber through the discharge port; and

b. increasing the size of the wall of the plenum chamber operating onthe hydraulic fluid as such plenum chamber contracts and maintaining thesubstantially constant force on the portion of the plenum chamberwhereby hydraulic means for contracting the first plenum chamber byapplying a substantially constant force to a portion of the first fluidis discharged at varying pressure levels from the plenum chamber throughthe discharge port. 7. A hydraulic power unit for discharging fluid atvarying pressure levels to a hydraulic device coupled to such powerunit, comprising:

a. a housing, such housing having formed therein a plenum chamber forstoring hydraulic fluid to be discharged, such wall of the plenumchamber;

c. means for varying the size of said portion of the first wall of thefirst plenum chamber;

(1. means for contracting the second plenum chamber by applying asubstantially constant force to a portion of the second wall of thesecond plenum chamber; and,

e. means connected to the first discharge port of the first plenumchamber being contractile and defined by a portion of such housing and awall slidable within said housing, such plenum chamber having adischarge port for providing at least a portion of the coupling betweenthe plenum chamber, the second discharge port of the second plenumchamber, and the hydraulic device for selecting which one of such firstand second plenum chambers is coupled to the hydraulic device. 12. Themethod recited in claim 6 wherein the size of the wall is increased.

plenum chamber and the hydraulic device; b. means for contracting theplenum chamber by applying a substantially constant force to a portionof such wall whereby hydraulic fluid is discharged at a first pressure

1. A hydraulic power unit for discharging hydraulic fluid at varyingpressure levels to a hydraulic device coupled to such power unit,comprising: a. a plenum chamber for storing hydraulic fluid to bedischarged, such plenum chamber being contractile and having a dischargeport, such discharge port providing at least a part of the couplingbetween the plenum chamber and the hydraulic device; b. means forcontracting the plenum chamber by applying a substantially constantforce to a portion of a wall partially defining such chamber wherebyhydraulic fluid is discharged at a first pressure level from the plenumchamber to the hydraulic device through the discharge port; and c.engagement means to connect said portion with the remainder of said wallto vary the size of said portion of the wall of the plenum chamber assuch plenum chamber contracts whereby hydraulic fluid is discharged at avarying pressure level from the plenum chamber to the hydraulic devicethrough the discharge port.
 2. A hydraulic power unit for discharginghydraulic fluid at varying pressure levels to a hydraulic device coupledto such power unit, comprising: a. a plenum chamber for storinghydraulic fluid to be discharged, such plenum chamber being contractile,partially defined by first and second slidable pistons and having adischarge port, such discharge port providing at least a part of thecoupling between the plenum chamber and the hydraulic device; b. meansfor sliding, with substantially constant force in a direction tocontract the Size of the plenum chamber, the first slidable pistonthrough a first and second distance; c. means for connecting the firstslidable piston to the second slidable piston during the seconddistance.
 3. A hydraulic power unit for discharging hydraulic fluid atvarying pressure levels to a hydraulic device coupled to such powerunit, comprising: a. a plenum chamber for storing hydraulic fluid to bedischarged, such plenum chamber being contractile, partially defined bya wall and having a discharge port formed therein, such discharge portproviding at least a part of the coupling between the plenum chamber andthe hydraulic device; b. a plurality of pistons forming such wall of theplenum chamber; c. meanS for contracting the plenum chamber by applyinga substantially constant force to one of such pistons whereby hydraulicfluid is discharged at a first pressure level from the plenum chamber tothe hydraulic device through the discharge port; and d. means forinterconnecting selected ones of such plurality of pistons as suchplenum chamber contracts whereby hydraulic fluid is discharged atcorresponding selectively varied pressure levels from the plenum chamberto the hydraulic device through the discharge port.
 4. A hydraulic powerunit for discharging hydraulic fluid at varying pressure levels to ahydraulic device coupled to such power unit, comprising: a. a plenumchamber for storing hydraulic fluid to be discharged, such plenumchamber being contractile and having a discharge port, such dischargeport providing at least a part of the coupling between the plenumchamber and the hydraulic device; b. first and second slidable pistonsforming a wall of the plenum chamber; c. means for sliding the firstslidable piston with substantially constant force in a direction tocontract the size of the plenum chamber a first, second and thirdportion; d. means for connecting the first slidable piston to the secondslidable piston during the second portion of the contraction of theplenum chamber; and, e. means for disconnecting the first slidablepiston from the second slidable piston during the third portion of thecontraction of the plenum chamber, whereby the hydraulic fluid storedwithin the plenum chamber is discharged at a first pressure level duringthe first and third portion of contraction of the plenum chamber and ata second pressure level during the second portion of contraction of theplenum chamber through the discharge port.
 5. A hydraulic power unit fordischarging hydraulic fluid at varying pressure levels to a hydraulicdevice coupled to such power unit, comprising: a. a first plenum chamberfor storing a first portion of hydraulic fluid to be discharged, suchplenum chamber being contractile and having a first discharge port, suchfirst discharge port providing at least a part of the coupling betweenthe plenum chamber and the hydraulic device; b. means for contractingthe first plenum chamber by applying a substantially constant force to aportion of the wall of the plenum chamber; c. means for varying the sizeof said portion of the wall of the first plenum chamber; d. a secondplenum chamber for storing a second portion of hydraulic fluid to bedischarged, such second plenum chamber being contractile and having asecond discharge port, such second discharge port providing at least apart of the coupling between the second plenum chamber and the hydraulicdevice; e. means for contracting the second plenum chamber by applying asubstantially constant force to a portion of the wall of the secondplenum chamber; and f. means connected to the first discharge port ofthe first plenum chamber, the second discharge port of the second plenumchamber, and the hydraulic device for selecting which one of such iscoupled to the hydraulic device.
 6. A method for discharging hydraulicfluid at varying pressure levels through a discharge port formed withina contractile fluid stoRing plenum chamber, the steps comprising: a.contracting the plenum chamber by applying a substantially constantforce to a portion of a wall partially defining the plenum chamberwhereby hydraulic fluid is discharged at a first pressure level from theplenum chamber through the discharge port; and b. increasing the size ofthe wall of the plenum chamber operating on the hydraulic fluid as suchplenum chamber contracts and maintaining the substantially constantforce on the portion of the plenum chamber whereby hydraulic fluid isdischarged at varying pressure levels from the plenum chamber throughthe discharge port.
 7. A hydraulic power unit for discharging fluid atvarying pressure levels to a hydraulic device coupled to such powerunit, comprising: a. a housing, such housing having formed therein aplenum chamber for storing hydraulic fluid to be discharged, such plenumchamber being contractile and defined by a portion of such housing and awall slidable within said housing, such plenum chamber having adischarge port for providing at least a portion of the coupling betweenthe plenum chamber and the hydraulic device; b. means for contractingthe plenum chamber by applying a substantially constant force to aportion of such wall whereby hydraulic fluid is discharged at a firstpressure level from the plenum chamber to the hydraulic device throughthe discharge port; and c. engagement means to connect said portion withthe remainder of such wall to vary the size of said portion of the wallof the plenum chamber as such plenum chamber contracts whereby hydraulicfluid is discharged at a varying pressure level from the plenum chamberto the hydraulic device through the discharge port.
 8. The hydraulicpower unit recited in claim 7 wherein: a. the wall is comprised of afirst and second piston, each such piston being slidable within suchhousing; b. the contracting means includes means for sliding the firstpiston, with substantially constant force in a direction to contract thesize of the plenum chamber, through a first and second distance; and c.the engagement means includes means for connecting the first piston tothe second piston as such first piston slides from the first distance tothe second distance.
 9. The hydraulic power unit recited in claim 7wherein: a. the wall is comprised of a plurality of pistons; b. thecontracting means includes means for applying a substantially constantforce to one of such pistons whereby hydraulic fluid is discharged at afirst pressure level from the chamber to the hydraulic device throughthe discharge port; and c. the engagement means includes means forinterconnecting selected ones of such plurality of pistons as suchplenum chamber contracts whereby hydraulic fluid is discharged atcorresponding varied pressure levels from the plenum chamber to thehydraulic device through the discharge port.
 10. The hydraulic powerunit recited in claim 8 wherein the area of the wall defined by thefirst piston is smaller than the area of the wall defined by the secondpiston.
 11. A hydraulic power unit for discharging hydraulic fluid at avarying pressure level to a hydraulic device coupled to such power unit,comprising: a. a housing, such housing having formed therein a first andsecond plenum chamber; i. such first plenum chamber having storedtherein a first portion of hydraulic fluid to be discharged, such firstplenum chamber being contractile, partially defined by a first wall andhaving a first discharge port, such first discharge port providing atleast a part of the coupling between the first plenum chamber and thehydraulic device; and ii. such second plenum chamber having storedtherein a second portion of hydraulic fluid to be discharged, suchsecond plenum chamber being contractile, partially defined by a secondwall and having a second discharge port providing at least a part of thecoupling between the second plenum chamber and the hydraulic device; b.meanS for contracting the first plenum chamber by applying asubstantially constant force to a portion of the first wall of theplenum chamber; c. means for varying the size of said portion of thefirst wall of the first plenum chamber; d. means for contracting thesecond plenum chamber by applying a substantially constant force to aportion of the second wall of the second plenum chamber; and, e. meansconnected to the first discharge port of the first plenum chamber, thesecond discharge port of the second plenum chamber, and the hydraulicdevice for selecting which one of such first and second plenum chambersis coupled to the hydraulic device.
 12. The method recited in claim 6wherein the size of the wall is increased.